1. Field of the Disclosure
The present disclosure relates to concentrates that can be diluted for use in wafer polishing applications. In particular, the present disclosure relates to a concentrate that can be diluted as much as 50× or more, while still maintaining optimal or near optimal polishing performance.
2. Description of the Related Art
The process known as chemical-mechanical polishing (CMP) involves polishing different layers on semiconductor wafers, using a polish pad and a slurry. Copper is a commonly used material for forming interconnects in semiconductor manufacturing. Once a copper inlaid structure is formed by, for example, a damascene process, the isolated copper wires are made by polishing and clearing copper and barrier metal between the inlaid wires. Copper and barrier layer CMP involves polishing of copper and barrier layers. It is desired to polish the wafers at a high removal rate of material to enhance throughput, while still maintaining favorable wafer characteristics such as a low number of overall defects.
A typical copper CMP process consists of 3 process steps. First, the electro-plated copper overburden (up to 2 um in thickness depending on technology node) is rapidly polished down at a relatively high down force, leaving some amount of copper until the deposition topography is fully planarized. (See FIG. 1) Throughput and planarization efficiency and low defects are key needs. Then, the remaining copper overburden after full planarization during the first step is polished off at a lower down force, with a stop on the barrier layer. The goal is to clear all copper from the barrier metal, but achieve significantly low dishing on the inlaid copper wire, with very low defects and improved surface roughness. Throughput is also important. This step can be combined with the first step, depending on the polisher type or configuration. Lastly, the thin barrier layer left after the second step, generally Ta or TaN, or both, is polished off with significant topography correction, low erosion and low defects. The slurry for to the first two steps may be the same or different. The barrier layer slurry, however, is usually a different composition.
Sometimes copper CMP slurries are made as concentrates. These concentrates have the benefit of being cheaper to make and ship, which reduces the cost of is ownership (COO) of the CMP slurry. The customer can simply add water and oxidizer at the point of use (POU), to form the POU slurry. One problem with this method, however, is that the concentrate must be properly designed to work well at the POU. By definition, a concentrate has much higher amounts of all the components than would be found in the POU slurry. However, it is not possible to make an unlimitedly high concentrated polishing composition, as would be preferable in a concentrate, because of stability and shelf life issues. In a colloidal slurry, stability is governed by particle surface effects, which depend on the type, amount, and chemistry of the particular particle. The higher the amount of abrasive in a slurry, the more the likelihood of instability. For example, if a POU polishing composition contains 1% abrasive, 1% removal rate enhancer, and 1% corrosion inhibitor, then a 10× Concentrate would be 10% abrasive, 10% removal rate enhancer, and 10% corrosion inhibitor, which could be highly unstable. Thus, CMP polishing compositions are made at a concentrate level where they are stable for at least 6 months of shelf life.
The disadvantage to these slurries, however, is that they can not be highly diluted (i.e. on an order of 10× or 20×), which adds to the cost of the CMP slurries ultimately needed for the polishing application. In addition, at higher dilutions, there is the risk that copper removal rates would be adversely affected, since at lower concentrations of abrasive and removal rate enhancer, one skilled in the art would expect the removal rate of copper to be less. The same holds true for any corrosion inhibitors used in the slurry—if the amount of corrosion inhibitor is diluted too much, the resulting slurry may not prevent corrosion of the copper inlays as much as is desired.
The prior art clearly shows that the more a concentrate is diluted, the more the performance of the resulting POU slurry will suffer. For example, U.S. Pat. No. 6,428,721, to Ina et al., lists several exemplary copper-polishing slurries in Table 1. The examples of that disclosure all comprise abrasive, hydrogen peroxide, alanine or glycine, and water. Table 1 clearly shows that the performance of the slurry drops off significantly as the slurry is diluted. When comparing Example 6 to Example 11, one can see that Example 11 is a 5× dilution of Example 6, since there is one-fifth as much abrasive in Example 11 as there is in Example 6. Consequently, Example 11 exhibits a drastically reduced removal rate of copper when compared to Example 6.
Another reference showing the relationship between copper removal rate and dilution is United States Patent Application Publication No. 2008/0254628, to Boggs et al. FIGS. 9 and 10, and the accompanying text in ¶¶123-124, very clearly illustrate that as the dilution of a CMP slurry increases, the copper removal rate drops off dramatically.
This relationship between dilution and polishing performance is also illustrated in the data sheet for the CoppeReady® Cu3900 slurry, made by DA Nanomaterials, and available at http://www.nanoslurry.com/datasheet/cu3900_product_sheet_final.pdf. The data sheet shows that when a slurry is diluted from a 4:1 strength to 9:1, the removal rate can be very severely affected, and can drop as much as 50%, depending on the downforce applied to the slurry.
Thus, there is a need for a concentrate that can be used in CMP slurries, which is stable, yet does not suffer from decreased performance when diluted to high levels, as this is very desirable from a COO standpoint.